1. Field of the Invention
This invention relates to a circuit that measures a physical quantity by detecting minute changes in the inductance of a coil used in such an apparatus as a magnetostrictive torque sensor, etc., and in particular, to a sensor circuit that can offset the effect of coil internal resistance, to perform high precision sensing of a physical quantity.
2. Description of the Related Art
An example of a sensor circuit for measuring a physical quantity by detecting minute changes in the inductance of a coil is described in the paper "Magnetostrictive Torque Sensor" by Yoshihiko UTSUI, et al published in the transactions of the magnetics society of the Institute of Electrical Engineers of Japan (reference number MAG-88-158, Oct. 11, 1988)
FIG. 3 is a block diagram of the conventional sensor circuit described in the above article (FIG. 4 in the referenced publication), and FIG. 4 shows the main parts of FIG. 3 (FIG. 5 in the referenced publication).
In FIG. 3, torque receiver shaft 1 is a rotating shaft, and first and second magnetic substances 2a and 2b are magnetostrictive layers affixed to the outer circumferential surface of torque receiver shaft 1.
As shown, first magnetic substance 2a is formed in multiple strips oriented at a fixed angle (=45.degree.) relative to the center axis of torque receiver shaft 1. Second magnetic substance 2b is separated from first magnetic substance 2a, along the length of the shaft, and is formed in multiple strips oriented at a fixed angle perpendicular to that of first magnetic substance 2a (=-45.degree.).
First and second coils 3a and 3b are separately positioned outside of and facing first and second magnetic substances 2a and 2b, respectively; and have inductances L1 and L2, and internal resistances r1 and r2, respectively, as shown in FIG. 4.
In FIG. 4, the first and second coils 3a and 3b, which are the sensor circuit coils, are connected in series with each other. Their inductance changes in response to a change in some physical quantity (such as a torque applied to torque receiver shaft 1).
In FIG. 3, drive circuit 4, connected across the series-connected combination of coils 3a and 3b, applies drive voltage Va to coils 3a and 3b to produce the coil current i.
Voltage detector circuit 5 outputs coil voltage Vc based on the voltages V1 and V2 generated across coils 3a and 3b by the application of drive voltage Va; e.g., the gain G times the differential of the voltages (V1-V2).
In response to a control signal C from drive circuit 4, phase detector 6 inputs coil voltage Vc, and outputs the result as detection voltage Vd.
Smoothing circuit 7 performs a smoothing process on detection voltage Vd to generate, as the final sensor output signal at the output terminal, a d.c. mean voltage Vm, responsive to changes in the inductances L1 and L2 of coils 3a and 3b, respectively.
Next, the operation of the conventional sensor circuit of FIGS. 3 and 4, with respect to the sensing of a physical quantity (e.g. torque) , will be explained, with reference to the waveforms of FIG. 5.
FIG. 5 shows the changes, over time, of various signal voltages occurring in the conventional sensor circuit in a magnetostrictive torque sensing system application. As shown, drive voltage Va is a rectangular waveform having an a.c. cycle time T. Va periodically switches between positive and negative levels centered around 0 V once each T/2 half cycle, a fixed time.
If a torque is applied to torque receiver shaft 1, a principal stress will be created in torque receiver shaft 1 in the direction of the two fixed angles (=.+-.45.degree.).
Due to the Villari effect, when this stress is created, the permeability of the magnetic strips 2a and 2b changes, with the permeability change being in one direction for tensile stress, and in the opposite direction for compression stress.
Accordingly, the inductance of one of the coils 3a and 3b will increase, and that of the other coil will decrease. This change in inductances L1 and L2 will cause a corresponding change in coil voltage Vc, and ultimately, in detection voltage Vd and mean voltage Vm. Thus the magnitude of the externally applied torque can be known by sensing the mean voltage Vm of detection voltage Vd.
In other words, as shown in FIG. 5, drive circuit 4 generates drive voltage rectangular waveform Va across the series combination of first and second coils 3a and 3b, causing coil current i to flow.
Now, if r1 and r2, the internal resistances of coils 3a and 3b, respectively, are assumed to be extremely low, then i, the coil current flowing in coils 3a and 3b, can be expressed in terms of L1 and L2, the inductances of coils 3a and 3b, respectively, by equation (1), below: EQU i=.intg.Va.multidot.dt/(L1+L2) (1)
And voltages V1 and V2 across the terminals of coils 3a and 3b, respectively, are given by equations (2) and (3), below: ##EQU1##
Voltage detector circuit 5 takes the difference between the above V1 and V2 voltages (=V1-V2), and outputs a coil voltage Vc equal to the voltage differential times the gain G, as expressed in equation (4), below: ##EQU2##
As is evident from equation (4), when L1 and L2, the inductances of coil 3a and 3b, respectively, are equal, the coil (differential) voltage Vc output by voltage detector circuit 5 will be zero, regardless of the value of drive voltage Va. If the inductances differ, however, the coil voltage Vc derived from the differential voltage will assume some non-zero level. If L2 is greater than L1, for example, the phase of the coil voltage Vc waveform will be opposite to that of the drive voltage Va, which is the case shown in FIG. 5.
At the same time as coil voltage Vc is being output by voltage detector circuit 5, drive circuit 4 outputs control signal C, which is synchronous with drive voltage Va (FIG. 5), and is also provided as an input to phase detector 6.
When control signal C is at a low level, phase detector circuit 6 outputs coil voltage Vc, as is, as a detection voltage Vd of the same polarity; and when control signal C is at a high level, phase detector 6 inverts coil voltage Vc, and outputs it as a detection voltage Vd of opposite polarity.
As a result, detection voltage Vd is a d.c. voltage that is directly proportional to the difference in the inductances of coils 3a and 3b (L2-L1).
Accordingly, if L2&gt;L1, then detection voltage Vd&gt;0; and if L2&lt;L1, then detection voltage Vd&lt;0. In other words, the magnitude and polarity of detection voltage Vd are functions of the magnitude and direction, respectively, of the physical quantity being sensed.
Now, since noise is superimposed on detection voltage Vd during phase detection, smoothing circuit 7 first performs a smoothing process on detection voltage Vd, after which it is output as d.c. mean voltage Vm.
The above describes the operation of an ideal sensor circuit, where r1 and r2, the internal resistances of coils 3a and 3b, are extremely small.
In actual sensor circuits, however, where small diameter wire is used in coils 3a and 3b to reduce physical size, the internal resistances r1 and r2 easily become too large to ignore.
Operation will now be described for a conventional sensor circuit having larger internal resistances r1 and r2, referring this time to FIG. 6.
In FIG. 6, Va, the drive voltage applied to the coils (=V1+V2), is separated into its resistive and inductive voltage components, Vr and VL. Thus, as shown in FIG. 6, along with drive voltage Va, coil current i, and control signal C, we also have Vr, the voltage developed due to the internal resistance of the coils (r1+r2), and VL, the voltage developed due to the inductance of the coils (L1+L2).
In this case, the drive voltage Va is the sum of Vr and VL.
If r1 and r2 (the internal resistances of the coils) can be ignored, as they were in the above description (FIG. 5), then the drive voltage Va waveform will exactly match the inductive component voltage VL waveform.
If coil internal resistances r1 and r2 are too large to be ignored, however, a resistive component voltage Vr, proportional to coil current i, is also developed, and a mismatch will exist between drive voltage Va and the inductive component voltage VL.
The larger r1 and r2 (the coil internal resistances), and the longer the a.c. cycle period T of the drive voltage Va, the more conspicuous will be this VL/Va mismatch.
If the a.c. cycle period T is made extremely long, for example, a voltage proportional to the coil internal resistance difference (r2-r1) will appear in the differential coil voltage Vc, and the sensor circuit will thus be incapable of accurately measuring the coil inductances L1 and L2.
If the internal resistances of the coils are precisely matched (r1=r2), this effect of internal resistances r1 and r2 will not appear in the differential coil voltage Vc. In a sensor circuit that measures physical quantity by sensing variations in inductances L1 and L2, where small diameter coil wire is used as discussed earlier, however, significant variances in internal resistances r1 and r2 are unavoidable.
Accordingly, where coils 3a and 3b are mass-produced, it is extremely difficult to maintain precisely matched r1 and r2 internal resistances in all of the parts produced.
Also, since the resistance of the coil wire varies with temperature, it is extremely difficult to eliminate the effect of internal resistances r1 and r2 on the differential coil voltage Vc over the entire temperature range. Consequently, an error is produced in the mean voltage Vm obtained as the final output, due to the effect of temperature induced variations of the internal resistances r1 and r2.
Conventional sensor circuits such as the one discussed above simply detected a differential coil voltage Vc in response to a control signal C synchronous with the timing of polarity inversions in drive voltage Va to obtain a detection voltage Vd, and also outputted a mean voltage Vm. Therefore, when coils having large internal resistances r1 and r2 were used, the circuits became incapable of accurately measuring the coil inductances L1 and L2, due to the presence of voltage Vr (FIG. 6) proportional to coil current i. This problem rendered the circuits incapable of accurately sensing a physical quantity.
The above problem is especially prominent when small diameter coil wire is used to reduce physical size. When small wire is used, variances in the internal resistances r1 and r2 become larger. Also, resistance varies with temperature. These combined factors make it extremely difficult to eliminate the effect of internal resistances rl and r2 on coil voltage Vc, and an error is therefore produced in the mean voltage Vm, making the circuit incapable of performing accurate sensing of a physical quantity.